1. Field of the Invention
This invention relates to a power unit which generates a single-phase AC power having a commercial frequency or a like frequency, and more particularly to a power unit of this kind which uses a cycloconverter having an input side thereof connected to a generator generating relatively small power and is constructed such that electric current to be supplied to a load connected to the power unit is limited when the power unit is in an excessively loaded state or the load is in a nearly short-circuited state.
2. Prior Art
Conventionally, a so-called cycloconverter is known, which directly converts AC power with a fixed frequency to AC power with another frequency.
Such a conventional cycloconverter is normally used for converting power supplied from a commercial frequency power line or power generated by a high power generator (see Japanese Patent Publication (Kokoku) No. 60-9429, for instance), and it is generally used for driving an AC electric motor.
In a conventional power unit using the cycloconverter described above, the cycloconverter is not provided with means for storing energy. Therefore, when the cycloconverter is used to obtain single-phase sinusoidal alternating current, the energy input to the cycloconverter also sinusoidally changes.
Therefore, when a small-sized generator which generates small power e.g. of several hundreds to several thousands kW is connected to the input side of the cycloconverter to generate a single-phase sinusoidal wave, only peak portions of the input sinusoidal waves can be utilized as input energy, so that the utilization efficiency is very low, resulting in only a very small output power being taken out as single-phase alternating current.
To overcome this inconvenience, a power unit has been proposed by the present assignee in Japanese Laid-Open Patent Publication (Kokai) No. 10-52047, in which as the load applied to the power unit increases, a desired waveform of a drive signal for driving the cycloconverter is changed from a sinusoidal waveform to a rectangular waveform which has maximum amplitude thereof limited by the output voltage of the power unit, to thereby keep the cycloconverter operating without difficulty even if an excessive load is applied to the power unit.
However, the conventional power unit is adapted to supply electric current to a load connected thereto under any excessive load condition according to the degree of the applied excessive load, so that even when there is no need to supply electric current to the load, judging from the load condition, the power unit continues to supply the electric current to the load. The load condition under which there is no need to supply electric current to the load includes, for example, an excessive load condition close to a short circuit of the load (including a short-circuited state). When the load is in such a condition, it is not necessary to continue supplying a sufficient amount of electric current to the load.